Technologies like microelectronics, micromechanics and biotechnology have created a high demand in industry for structuring and probing specimens within the nanometer scale. On such a small scale, probing or structuring, e.g. of photomasks, is often done with electron beams which are generated and focused in electron beam devices like electron microscopes or electron beam pattern generators. Electrons beams offer superior spatial resolution compared to e.g. photon beams due to their short wave lengths at a comparable particle energy.
While electron beam devices can meet the spatial resolution requirements they are often too slow to deliver the throughput needed in large scale manufacturing. To overcome the throughput limitations, electron beam devices with multiple beams have been proposed with various designs. Such electron multiple beam devices with higher electron beam density usually rely on arrays of field emission cathodes where the field emission cathodes are integrated onto a substrate. Such field emission cathode arrays are fabricated by using micromechanical or microelectronic fabrication techniques. They usually comprise an array of emitter tips and an array of extracting electrodes with extracting electrode and emitter tip facing each other one to one. In electron beam devices with such an array of emitter tips, a single pixel of an image to be written on the surface of a sample may be represented by a single emitter tip.
During pattern writing or inspection, the electron beam is moved over the surface of the sample in substantially parallel write scan lines. An electron-sensitive resist deposited on the sample surface is partly exposed to the electron beam. The electron beam changes the molecular composition of the resist, thus making it soluble in a developing solution, if the exposure dose, i.e. the temporarily integrated beam current, is sufficiently large. Subsequently, the exposed portions of the resist are removed and the layers underlying the resist can be etched in the regions where the resist has been removed.
The beam current of the electron beam varies with position since only predetermined pixels on the write scan line are to be exposed to write a predetermined pattern. Therefore, the electron beam has to be turned on for writing and turned off for not writing a pixel. This switching has to be done very fast to achieve a reasonable throughput. One way of switching on and off the electron beam is to switch on and off the emitter tip itself which is done by switching the tip-to-gate voltage, i.e. the voltage difference between the emitter tip and the gate electrode, from a low “tip off” voltage level to a high “tip on”voltage level. The ratio of the “tip off” current to the “tip on” current is called the extinction ratio.
In known electron beam apparatus, the extinction ratio is about 10−2, i.e. the “tip on” current is about 102 times larger than the “tip off” current. For considerably larger extinction ratios, a variation of the critical dimension (CD) could occur since the residual “tip off” current still contributes to the exposure dose and thereby increases the line width of the pattern. This effect is most critical at locations where the electron beam spot is moving slowly across the sample surface, i.e. especially during the write scan turn at the beginning or end of each new write scan line. The temporarily integrated exposure dose of the “tip off” current may become too large in the turn regions and lead to variations of the critical dimension (CD) or other undesired results. Therefore, some apparatus known in the art use an additional blanker, i.e. a device for electrically deflecting the electron beam away from the sample to avoid these CD variations.
Furthermore, the voltage swing of the emitter tip has to be relatively high, e.g. several tens of volts for present tip geometries to achieve the desired extinction ratio. This relatively high voltage swing causes the “tip on”/“tip off”switching of emitter tips to be slow and, consequently, limits the throughput. Furthermore, the heat load of interconnects and integrated electronics as well as the in-vacuum power dissipation are relatively high since the tip current depends approximately exponentially on the tip-to-gate voltage.